1. Field of the Invention
The present invention relates to an apparatus and a method for processing photographic material, and in particular to the development of photographically exposed plates in order to prepare them for use as printing plates in a printing operation.
Photographic materials and photographic exposure, respectively, as used herein and in the following refer to all kinds of photographic materials, related materials, and processes, respectively, irrespective of whether they are based on the use of visible light or of another form of radiation as long as they serve to reproduce an image by similar general principles. Sheets herein comprise all flat objects that may be conveyed as a web along a path, e.g. foils, laminates, plates whether in short formats, long formats or continuous webs. The sheets may be flexible or inflexible. The printing operations may be offset printing, flexographic printing, or other. Treatment by liquid may comprise reacting an emulsion on the face with chemicals in order to fix an image, to dissolve or to swell parts of the emulsion in order to clean the surface, or in order to provide the surface with a finishing, etc. as known in the art of photographic image processing.
2. Description of the Prior Art
EP-A 0 410 500 discloses a lithographic printing plate adapted for offset printing in which a pattern is formed in the printing plate by a photographic process. This pattern is formed by coating the plate with a light sensitive emulsion, exposing said emulsion and developing the image of the exposed emulsion. The developing process comprises different baths through which the plate passes successively and which have different purposes, such as fixation of the exposed image, washing off of chemicals, and surface treatment of the finished plate.
EP-A 0 623 854 discloses an apparatus for processing a lithographic offset printing plate in such a process, which apparatus comprises different baths through which the plate is conveyed, controlled by suitable guide rollers. The guide rollers define a conveyor path guiding the plate down into the respective baths and up again, the plate being flexed in order to follow this course.
EP-A 0 415 392 discloses a processing apparatus for processing a printing plate, provided with a bath and guide rollers which control the plate so that it is descended obliquely into the solution in the bath, flexed to follow a curved passage below the solution level and pulled upwardly out of the bath. The apparatus comprises a rotating brush roller, accommodated submerged in the solution, and a floating cover for shielding a portion of the surface of the solution, including the area above the brush. The purpose of the floating cover is to counteract the deterioration of the processing solution by the air.
EP-A 0 050 818 discloses a processing apparatus for printing plates provided with a tank and with conveyor means for guiding the plate down into the bath along a curved passage and up again, which apparatus comprises a rotating brush partially submerged in the bath and adapted for brushing the upper surface of the plate. This prior art apparatus is provided with a protective shield above the rotating brush, the rear edge of said protective shield being disposed completely below the solution level. The purpose of this is to counteract foaming in the processing solution and to counteract deterioration of the processing solution by the air.
EP 0 260 615 discloses a processing apparatus for printing plates, comprising conveyor means for conveying the printing plate along a straight path descending into a bath to exit the bath at a lower portion thereof through the nip between a pair of rollers adapted for holding back the liquid. A rotating, partially submerged brush roller brushes the upper face of the printing plate.
U.S. Pat. No. 4,515,456 discloses a processing apparatus adapted for conveying the plates along a straight path through a bath, wherein sealing of entry and exit ports are provided by supple lip seals forced against the face of the plate.
Brushing generally enhances the processing process, but generally has an adverse effect on the useful life of many developer baths. Many processing chemicals are highly reactive and may exchange different gasses with the air, e.g. absorb CO.sub.2 or oxygen which react with chemicals in the baths so that these no longer have the desired effect and therefore must be reactivated or replaced. Movable brushes stirring the surface of the solution strongly accelerate this deterioration process.
For reasons of minimizing handling of the delicate emulsion it is generally favored to design the processing apparatuses for conveying the plates with the emulsion-covered side facing up.
An apparatus provided with brushes for brushing the sheet upper face, which brushes are completely submerged below the solution surface, is limited in its scope of design, which restricts the use and functionality of the apparatus. If the apparatus is to be designed with conveyor means for conveying the plates downwards and deep into the bath and subsequently upwards to exit the bath, and if the bath is not very long, the plate must be flexed by a relatively acute radius of curvature. This strains the plates and causes resistance to the motion. Furthermore, as a result of the small radius of curvature, the distance between the plate and the brush and hence the force of engagement by the brush may vary depending on several factors, among which the size, resilience and thickness of the plate. Due to the delicate nature of the emulsion on the surface to be brushed, the force of engagement by the brush is, however, a very critical parameter, and an apparatus of the above type is therefore not particularly suitable for processing plates of different grades and thickness.